1. Field of the Invention
The invention relates generally to an apparatus and method for detecting abnormal air-fuel ratio variation among cylinders of a multi-cylinder internal combustion engine. More specifically, the invention relates to an apparatus and method for detecting relatively great air-fuel ratio variation among cylinders of a multi-cylinder internal combustion engine.
2. Description of the Related Art
In an internal combustion engine provided with an exhaust gas control system that uses a catalyst, it is usually necessary to control a mixture ratio between air and fuel, which constitute an air-fuel mixture that is burned in the internal combustion engine, that is, an air-fuel ratio, in order to remove toxic substances in the exhaust gas using the catalyst with high efficiency. To control the air-fuel ratio, an air-fuel ratio sensor is provided in an exhaust passage of the internal combustion engine, and feedback control is executed so that the air-fuel ratio that is detected by the air-fuel ratio sensor matches a predetermined target air-fuel ratio.
In a multi-cylinder internal combustion engine, air-fuel ratio control is usually executed using the same control amount for all the cylinders. Therefore, even if the air-fuel ratio control is executed, the actual air-fuel ratio may vary among the cylinders. If the variation range is narrow, such small air-fuel ratio variation is absorbed by executing the air-fuel ratio feedback control, and toxic substances in the exhaust gas are removed by the catalyst. Therefore, such small air-fuel ratio variation does not exert an influence on the exhaust emission, and, therefore, does not cause a problem. However, if the air-fuel ratio greatly varies among the cylinders due to, for example, a malfunction of a fuel injection system of part of the cylinders, the exhaust emission deteriorates, which may cause a problem. Preferably, such great air-fuel ratio variation that may cause deterioration of the exhaust emission should be detected as an abnormality. Especially, in the case of an internal combustion engine for an automobile, detecting abnormal air-fuel ratio variation among cylinders using an on-board device is required in order to prevent a vehicle that emits deteriorated exhaust emission from running. Recently, there are moves for legislating for provision of an on-board device that detects abnormal air-fuel ratio variation among the cylinders.
Japanese Patent Application Publication No. 04-318250 (JP-A-04-318250) describes an apparatus which determines that a fuel supply system malfunctions when an air-fuel ratio feedback correction coefficient that is used in air-fuel ratio feedback control is equal to or larger than a predetermined value. The apparatus is able to determine that some sort of malfunction has occurred somewhere in the fuel supply system. However, the apparatus is not able to detect abnormal air-fuel ratio variation, that is, abnormal deviation of the air-fuel ratio in at least one cylinder from the air-fuel ratio in the other cylinders.
Japanese Patent Application Publication No. 2000-220489 (JP-A-2000-220489) describes an engine control apparatus that calculates air-fuel ratios in respective cylinders and individually controls the air-fuel ratios in the cylinders, in a multi-cylinder engine in which a single air-fuel ratio sensor is arranged in an exhaust pipe gathering portion of the engine. The engine control apparatus calculates an air-fuel ratio based on a signal output from the air-fuel ratio sensor, analyzes the calculated air-fuel ratio into frequency components in a predetermined range, and estimates the air-fuel ratios in the respective cylinders based on the analyzed frequency components.
If the air-fuel ratios in the respective cylinders are estimated using, for example, the apparatus described in JP-2000-220489, it may be possible to determine whether abnormal air-fuel ratio variation among the cylinders has occurred by comparing the air-fuel ratios with each other. However, with the apparatus described in JP-A-2000-220489, it is necessary to detect the fluctuations of the air-fuel ratio that is in synchronization with engine rotation in short cycles using the air-fuel ratio sensor. Therefore, a considerably highly-responsive air-fuel ratio sensor is required. Even when such an air-fuel ratio sensor is available, if the sensor deteriorates and therefore the response becomes slower, the sensor may fail to function properly. In addition, a high-speed processing data sample and a powerful ECU are required. It is difficult to separate fluctuations of the air-fuel ratio and noise from each other to detect only the fluctuations of the air-fuel ratio using a highly-responsive sensor. Further, restrictions are imposed on the engine operating condition, for example, the engine operating condition is restricted to the steady operating condition to minimize disturbance. It is preferable to arrange the sensor at a position as close as possible to a combustion chamber in order to detect the fluctuations of the air-fuel ratio that is in synchronization with engine rotation. However, in this case, a crack may be caused in a sensor element due to moisture in the exhaust gas. Therefore, it is necessary to select an exhaust manifold having an appropriate shape and to arrange the sensor at an appropriate position so that the gas contacts the air-fuel ratio sensor appropriately. As described above, even if the air-fuel ratios in the respective cylinders are estimated to determine whether abnormal air-fuel ratio variation among the cylinders has occurred, there are many problems to be solved. Therefore, it is difficult to actually implement the above-described technology.